In the manufacture of semiconductor devices and integrated circuits it is necessary to modify the semiconductor substrate material (particularly silicon) by diffusing or implanting therein atoms or molecules of selected dopants to produce regions in the semiconductor substrate of selected varying conductivity and having majority charge carriers of different polarities. Typical dopant materials used in this process are boron, phosphorus, arsenic and antimony.
Doping the semiconductor substrate using ion implantation has become increasingly important with the continuing reduction in feature sizes on integrated circuit structures.
When implantation apparatus is arranged to implant boron ions, the standard prior art arrangement for generating these ions is to feed boron trifluoride (BF.sub.3) gas as a feedstock into an arc chamber. In the arc chamber, a plasma is produced in which the BF.sub.3 molecules are cracked and ionised to produce B.sup.+, BF.sup.+ and BF.sub.2.sup.+ ions. These ions are extracted from the arc chamber and accelerated to a predetermined energy at which they are passed through a mass selection arrangement. The mass selection arrangement typically comprises a magnetic field in which the radius of curvature of the flight path of the ions from the source will be dependent upon the mass/charge ratio of the individual ions. A mass selection slit at the exit of the magnetic field region allows ions of a selected mass/charge ratio to pass through to the target substrate.
Prior to implantation, the semiconductor substrate i.e. typically a silicon or gallium arsenide wafer, is prepared with a required pattern of photoresist, so that the ions will be implanted only in selected regions of the wafer as required. The depth to which ions are implanted in the wafer is dependent upon the energy of the ions as they impinge upon the wafer surface. With the increasing demand for smaller and faster semiconductor devices, there is an increasing need for the production of very shallow structures in the wafer requiring the use of ions of relatively low energy at the point of implantation.
On the other hand, there is still a need for the flux of ions impinging upon the wafer (at the desirable low energies) to be as high as possible, implying a relatively high beam current density of the ions. This is required in order to provide high wafer processing speeds.
The requirements of high ionic beam current density and low energy at the point of implantation are conflicting. With very low implant energies, it becomes increasingly difficult to control the ion beam and avoid a substantial loss of ions from the beam, for example because of dissipation through space charge effects.
In prior art boron ion sources the ion current extracted from the source is directly proportional to the extraction energy up to a saturation energy of about 40 keV. For implantation energies below 10 keV, it has been proposed to extract ions from the source at 10 keV or higher and then decelerate the ions further down the beam line before the ions impinge upon the target. However, even when operating the implantation apparatus with the ion source at saturation extraction energy, the net current of mass selected, ions impinging upon the wafer may be less than desirable.
It should be understood here that when using BF.sub.3 as the feedstock gas for the ion source, not only B.sup.+ ions but also BF.sub.2.sup.+, BF.sup.+, and F.sup.+ are produced in the source and duly extracted. The mass selection arrangement ensures only the desired ions, usually B.sup.+, are fed onto the target, so that the part of the extracted beam current represented by the non-desired ions is lost. In general, there is a need to maximise the beam current impinging on the wafer at all implantation energies.
Boron halides, in particular BF.sub.3, are very poisonous and United Kingdom Patent No. 1442586 discloses the use of boron oxide as an alternative non-poisonous feed material for boron ion sources. However, the presence of oxygen in the ion source is highly undesirable as it severely limits cathode life. This United Kingdom patent also states that elemental boron is unsuitable as a feed stock since it provides insufficient vapour pressure at conventional oven temperatures.
International Patent Publication WO93/23869 discloses the use of boron powder in a special arrangement of ion source in which the boron powder is directly exposed to the plasma in the arc chamber. The boron powder is biased more negatively than the cathode in the source to promote an intense secondary discharge at the surface of the boron powder. In addition BF.sub.3 gas is bled through the powder into the arc chamber. The International Publication teaches that the use of boron vapour from elemental boron in a conventional arc chamber is not suitable as the arc chamber itself then has to be maintained at about 2000.degree. C. to prevent condensation of boron on the walls of the arc chamber.